Kjob: add elapsedTime() returns the ms the job ran

This requires sub-class to add a startElapsedTimer() call at the beginning of their start function to work.

Added a todo KF7 since with with a simple API change this limitation can be lifted.

The use for this, is to have accurate average speed measurement throughout jobs and not just relying on speed() that is real an "instant" speed, instead of an average. This will allow also to say things like "Downloaded in 1s seconds at 1Mbit/s" in dialogs making it easier to log, debug, inspect and report bugs related to performance.

This is primarily meant to help implement plasma/plasma-workspace!4851 Next step is adding the unfortunately necessary startElapsedTimer() in KIO jobs. Then transmit this on the wire of KJobTrackerInterface